Certainly, the human-animal distinction is still workable; after all, we rarely make errors in assigning entities to one category or the other. But after Darwin, the distinction suddenly seems arbitrary – as arbitrary as the equally workable distinction between, say, turtles and non-turtles.
(pp. 154-155) - Darwin, God and the Meaning of Life: How Evolutionary Theory Undermines Everything You Think You Know (2010)